[SymbolTable][Perf] Use hash_combine instead of a custom hash, also use memcmp.

ArrayRef<uint8_t>::equals(); lowers to a byte compare loop :(.

TODO: Figure out if we are getting hash collisions, or just have a lot of equal
content. Also test if crypto hashing the content instead of full compare is
better.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@177588 91177308-0d34-0410-b5e6-96231b3b80d8